This invention is directed to well drilling and, in particular, to processes and devices for well drilling wherein a wellbore is advanced with a drill bit affixed to the distal end of a casing string.
The drilling of wells, for example, for oil and gas production conventionally employs relatively small diameter strings of drill pipe to which is secured a drill bit of somewhat larger diameter. After a selected portion of the wellbore has been drilled, the well bore is usually lined with a string of tubulars known as casing. The casing (herein used to encompass any wellbore liner) has a larger diameter than drill pipe and a smaller diameter than the drill bit. This conventional system which requires sequentially drilling the borehole using drill pipe with a drill bit attached thereto, pulling the drill pipe out of the borehole and running casing into the borehole is time consuming and costly. In addition, control of the well is difficult during the period that the drill pipe is being removed and the casing is being run in.
Drilling with casing is gaining popularity as a method for drilling wherein the casing is used as the drilling conduit and, after drilling, the casing remains downhole to act as the wellbore liner. A drilling assembly, including a drill bit and one or more hole enlargement tool such as, for example, an underreamer, is used which drills a borehole of sufficient diameter to accommodate the casing. The drilling assembly is deployed on the advancing end of the casing. The drill bit can be retractable and/or removable through the casing.
Casing drilling has been tested for drilling vertical, straight wellbores. However, new techniques for reservoir management require the drilling of curved, directional boreholes. This technique is commonly termed directional drilling or horizontal drilling, where a well bore close to horizontal is formed, and can be used to create boreholes having radii of curvature ranging from tens, hundreds or thousands of feet. Various techniques have been developed for drilling directional boreholes including the use of whipstocks.
Of particular importance in directional drilling are rotary steerable tools or downhole motors equipped with bent housings and/or bent subs which permit control of forces acting perpendicular to the drill string to steer the drill bit in a selected direction while drilling. To date, directional drilling systems have been developed for use with conventional drill pipe. No system is currently available for drilling directional boreholes using casing. This causes drillers to resort to the conventional system of first drilling the borehole and then, separately, lining it. When directional drilling, companies must accept the increased cost, time and hazard of separately drilling and then lining a borehole.
A method and apparatus for drilling directional boreholes using casing has been invented. The present invention provides a method and apparatus for drilling a directional borehole wherein the drill string is formed of casing which can be left in place after drilling is complete to act as the borehole liner. By utilizing casing as both the drilling conduit and the wellbore liner, the expensive and hazardous drill string insertion and retrieval operations are minimized.
In accordance with a broad aspect of the present invention, there is provided an apparatus for drilling a wellbore in an earth formation comprising: a drill string having a longitudinal bore therethrough; a drilling assembly connected at the lower end of the drill string and selected to be retrievable through the longitudinal bore of the drill string; and a directional borehole drilling assembly connected to the drill string and including biasing means for applying a force to the drill bit to drive it laterally relative to the wellbore.
The drill string useful in the present invention must have a longitudinal bore of sufficient inner diameter and be of a form suitable to act as a wellbore liner. In one embodiment, the drill string is casing.
At the lower end of the casing is mounted a drilling assembly selected to be operable to form a borehole having a diameter greater than the diameter of the casing while including a portion which is retrievable through the longitudinal bore of the drill string to provide for removal of the portion without removing the drill string of casing. The drilling assembly can be mountable to the casing in any suitable way, for example, by toothed engaging pads, corresponding locking dogs or latches, packers or other means. The drilling assembly can be any suitable assembly for drilling a borehole including, for example, rotary bits, impact bits or laser technology. In one embodiment, the bit assembly includes a primary bit and a hole enlargement tool. The hole enlargement tool or tools is/are positioned to enlarge the wellbore behind the primary bit. In one embodiment, the hole enlargement tool is one or more underreamers. To permit retrieval of the drilling assembly including underreamers, they can be radially retractable and extendable. The underreamers can be extendable in various ways, such as for example by pivotal movement or by sliding movement. Another drilling assembly useful in the present invention is a bicentre bit which does not have retractable underreamers but instead has an eccentric cutter positioned so that the drilling assembly can be shifted within the inner diameter of the drill string to permit it to be retrieved through the longitudinal bore of the drill string.
The bit assembly can be suitable for use in rotary drilling, wherein rotation is imparted to the drill bit by rotation of the drill string, for example, from surface. Alternately, the drilling assembly can be suitable for use in motor drilling wherein the drill bit is driven to rotate by a downhole drive unit such as a Moineau-type motor, a vane motor, a turbine motor or an electric motor.
A directional borehole drilling assembly useful in the present invention includes biasing means for applying a force to the drill bit to drive it laterally relative to the wellbore. In one aspect of the invention, the directional borehole drilling assembly is useful in motor drilling and, in another aspect, the directional borehole drilling assembly is useful with a rotary drilling system. The biasing means can be any suitable means for deflecting the drill bit to drill a curved borehole.
In one embodiment for use in motor drilling, the biasing means is a bent sub or a bent housing. The bent sub and bent housing each have an upper section and a lower section and a connector disposed between the upper section and the lower section to attach the upper section to the lower section, the connector being selected to provide for the lower section to be out of axial alignment with the upper section. The connector can be any suitable means including, for example, a bent section in a mud motor housing, a bent pipe section, a flexible joint or any other connector for mounting the lower section such that its longitudinal axis can be offset from the longitudinal axis of the upper section. The upper section can be a section of the drill string or another section such as, for example a tube section of any desired length. The lower section is any desired member such as, for example, a drill collar, a cross-over sub, formation evaluation tools or a section of drill string of any desired length. In a bent housing, the upper section and the lower section are often sections of the mud motor housing. Outer collars, eccentric members, razor backs and/or other directional drilling means can be mounted on the upper section, lower section, bit or casing, as desired.
In an embodiment for use in rotary drilling, wherein rotation is imparted to the drill string in order to effect borehole formation, the biasing means can be, for example, a fulcrum assembly such as an eccentric member positioned about the drill string, a hydraulic or non-hydraulic modulated biasing means or a drilling fluid jetting system.
A hydraulic or non-hydraulic modulated biasing means has moveable thrust members or pads which are displaceable outwardly at the same selected rotational position in the wellbore during each rotational cycle of the drill string to bias the drilling assembly laterally and, thereby, to control the direction of drilling.
In a drilling fluid jetting system, the biasing means is a jet of fluid discharged under the control of a valving system. The valving system controls the discharge of drilling fluid into the borehole either in a evenly distributed manner, to drill straight, or into a selected sector of the borehole during each rotational cycle of the drill string when it is desirable to divert the drill bit to drill in another direction.
Where desired, at least a portion of the directional borehole drilling assembly is retrievable through the drill string. In particular, the bit, the upper section and the lower section can be sized and/or formed to be retrievable through the drill string separately or as a unitary member.
In accordance with another broad aspect of the present invention, there is provided a method for directionally drilling a well with a well casing as an elongated tubular drill string and a drilling assembly retrievable from the lower distal end of the drill string without withdrawing the drill string from a wellbore being formed by the drilling assembly, the method comprising: providing the casing as the drill string; providing a directional borehole drilling assembly connected to the drill string and including biasing means for applying a force to the drilling assembly to drive it laterally relative to the wellbore; providing a drilling assembly connected at the distal end of the drill string and being retrievable through the longitudinal bore of the drill string; inserting the drill string, the directional borehole drilling assembly and the bit assembly into the wellbore and driving the bit assembly to rotate for cutting the wellbore to a diameter greater than the diameter of the drill string; operating the biasing means to drive the drilling assembly laterally relative to the wellbore; removing the bit assembly from the distal end of the drill sting upon completion of the wellbore without removing the drill string from the wellbore; and leaving the drill string in the wellbore to serve as the casing for the well.